Novel Process for Removing Sulfur from Fuels

ABSTRACT

A process for removing sulfur-containing compounds from fuel, said process comprising contacting the fuel in liquid phase with air to oxidise the sulfur containing compounds, said contacting being carried out in the presence of at least one transition metal oxide catalyst.

This invention relates to a novel process for removing sulfur-containingorganic compounds from fuels by oxidative desulfurisation.

For many years, growing concerns over environmental pollution caused bythe presence of sulfur-containing compounds in hydrocarbon-based fuelssuch as diesel, gasoline, and kerosene has provided impetus for thedevelopment of desulfurisation technology. A high level of sulfur infuels is undesirable due to the formation of SO_(x) from the combustionof sulfur-containing compounds. SO_(x) in turn causes acid rain to form,leading to widespread damage to buildings and disturbing delicatebalances in the ecosystem. Furthermore, sulfur compounds in fuels poisonthe noble metal catalysts used in automobile catalytic converters,causing fuel to be incompletely combusted and thus result in theemission of incompletely combusted hydrocarbons, carbon monoxide,nitrogen oxides in the vehicle exhaust, all of which are precursors ofindustrial smog.

To protect the environment against pollution caused by sulfur,governmental agencies have set up guidelines for petroleum refiningcompanies to limit the level of sulfur in commercial fuels. For example,the United States Environmental Protection Agency (EPA) has recentlyannounced plans to reduce sulfur content of diesel fuels from thecurrent 500 parts per million (ppm) to 50 ppm in 2006.

The industrial removal of sulfur from fuels is generally carried out viathe well-established hydro-desulfurization (HDS) process, described forexample in the GB Patent 438,354. HDS involves the catalytic treatmentof fuel with hydrogen to convert sulfur-containing compounds to hydrogensulfide, H₂S. H₂S is in turn converted to elemental sulfur by the Clausprocess. For low point and middle boiling point distillates, the typicalHDS reaction requires relatively severe conditions of about 300° C. to400° C. and 0.7 to 5 MPa.

It has been found that HDS is less effective in removing certainresidual sulfur-containing compounds present in petroleum distillates,particularly heterocyclic sulfur-containing compounds such asthiophenes, benzothiophenes (BT), dibenzothiophenes (DBT), especiallyDBTs having alkyl substituents on their 4 and/or 6 positions (Ind. Eng.Chem. Res. 2002, 41, 4362-4375), as well as higher homologs of thesecompounds. One possible reason is that the sulfur atom is stericallyhindered by the bulky benzyl groups, thereby making the sulfur atom lessaccessible to oxidative attack.

Although these heterocyclic sulfur compounds may be removed byoptionally increasing the severity of HDS reaction conditions, the onsetof other side reactions leading to the formation of coke, degradation ofthe octane level of the fuel, as well as the accompanying increase inenergy and hydrogen consumption, makes the HDS option undesirable froman economic perspective.

Therefore, alternative processes have been developed to further lowersulfur content of fuels through the removal of residualsulfur-containing compounds from processed fuels, while maintaining orimproving fuel performance. The term “deep desulfurisation” is typicallyapplied to such processes.

In general, deep desulfurisation is carried out on fuels which havealready undergone HDS and thus have sulfur contents that have beenlowered from the initial level of several thousand ppm to severalhundred ppm. Deep desulfurisation is thus distinguished fromconventional HDS in that it the oxidation of sulfur occurs at a sulfurconcentration that is by comparison much lower. From the perspective ofreaction kinetics, reactions that are first order or higher with respectto the reactant become more difficult to carry out as the concentrationof the reactant becomes gradually lower.

One current approach to the deep desulfurisation of fuels include theuse of transition metal adsorbents for removing the sulfur compounds, asdisclosed in US Patent Application No. 2004/0007506, for example.

Another approach that has been investigated is oxidative desulfurisation(ODS), in which fuel is contacted with oxidants such as hydrogenperoxide, ozone, nitrogen dioxide, and tert-butyl-hydroperoxide, inorder to selectively oxidise the sulfur compounds present in the fuel topolar organic compounds. These polar compounds can be easily separatedfrom the hydrophobic hydrocarbon based fuel via solvent (liquid)extraction using solvents such as alcohols, amines, ketones oraldehydes, for example.

U.S. Pat. No. 3,847,800 discloses an ODS process in which nitrogendioxide gas is used as an oxidant to oxidise sulfur-containing compoundsin diesel fuel. Methanol and ethanol are subsequently used asnon-miscible solvents for extracting the oxidised compounds.

The European Patent Application No. EP 0 565 324 A1 discloses a methodof recovering organic sulfur compounds from liquid oil. The methodinvolves a pure redox-based process between the sulfur compounds and theoxidant. The liquid oil to be processed is treated with an oxidisingagent, such as ozone gas, chlorine gas, peracetic acid or hydrogenperoxide to oxidise the sulfur compounds in the oil into sulfones orsulfoxides. Subsequently, the oxidised products are separated using acombination of means such as distillation, solvent extraction andadsorption.

The use of gaseous or liquid oxidants such as hydrogen peroxide, ozone,dioxirane and ethylene oxide to convert the sulfur compounds present infuels into sulfones is also disclosed in U.S. Pat. No. 6,160,193. Theoxidants are contacted with fuel in liquid phase, and the oxidisedproducts thus formed are subsequently extracted from the fuel by addingdimethyl sulfoxide to the reaction mixture. According to this patent,when hydrogen peroxide is used as oxidant, metal catalysts can be usedto accelerate the decomposition of the hydrogen peroxide to form thereactive oxidising species. The dimethyl sulfoxide forms an aqueousphase which is separable from the hydrocarbon phase by gravityseparation or centrifugation. Oxidation is reportedly carried out atabout 30° C. to 100° C. at pressures of about 150 psig (about 12.5 bars)or preferably at a pressure of 30 psig (about 2.5 bars).

U.S. Pat. No. 6,402,940 further discloses a method for the oxidativeremoval of sulfur using an oxidising aqueous solution that compriseshydrogen peroxide and formic acid in specific molar ratios. Thisoxidising solution is mixed with liquid fuel at temperatures of 50 to130° C., thereby oxidising the sulfur compounds into polar compounds.The polar compounds are subsequently removed by simple extraction andphase separation.

Finally, the PCT application WO 03/051798 discloses a method forcarrying out ODS in which the fuel and oxidant are contacted in thegas-phase. The fuel is first vapourised and then contacted with asupported metal oxide catalyst in the presence of oxygen. Sulfur isliberated from hydrocarbon molecules in the fuel as sulfur dioxide gas,which is subsequently removed with an ion exchange column.

Nevertheless, despite the developments that have taken place,alternative technologies need to be developed in order to reduce sulfurcontent in fuels while preferably maintaining/improving fuel performancewithout significant capital and operating costs. Accordingly, it is anobject of the present invention to provide a corresponding process forremoving sulfur-containing compounds in fuels in order to obtain fuelsthat have low sulfur content. It is a further object of the invention toprovide a process for the effective removal of sulfur compounds fromfuels which are not easily removed through conventional HDS processes,but is still economical to carry out on an industrial scale.

This object is solved by a process for removing sulfur-containingcompounds from fuel, comprising:

-   -   contacting the fuel in liquid phase with air to oxidise the        sulfur-containing compounds, said contacting being carried out        in the presence of at least one transition metal oxide catalyst.

In oxidative desulfurisation processes, the removal of sulfur-containingcompounds from petroleum-based hydrocarbon fuels is carried out byoxidising the sulfur-containing compounds using a suitable oxidant. Thesulfur containing compounds are converted into compounds havingincreased polarity relative to the fuel, and then subsequentlyextracted. In the present invention, oxidation is accomplished bycontacting liquid fuel with air in the presence of transition metaloxide catalysts that selectively facilitates the oxidation of theresidual sulphur compounds.

One advantage of the invention comes from the use of gaseous oxygenfound in air. While costly oxidants such as hydrogen peroxide or ozoneare required in some of the current desulfurisation processes, thepresent process only requires the use of air as oxidant. Since air isabundant and freely obtainable from the atmosphere, the present processcan be carried out very economically. The use of air also eliminates theneed to carry out any oxidant recovery process that is usually requiredif liquid oxidants such as hydrogen peroxide are used. Another advantageof the inventive process comes from treating fuel in liquid phase, whichallows mild process conditions (low process temperatures and pressures)to be used for the efficient oxidation of sulfur compounds, as comparedto other desulfurisation processes known in the art in which more severeconditions are needed. Mild process conditions also mean that energyconsumption for the process is low, thus resulting in further costsavings. Yet another advantage of the present invention is the ease ofintegration into any existing refinery for the production of diesel, asafforded by the mild process conditions of liquid phase contacting andthe use of air. Furthermore, the use of a selective oxidation catalystalso permits the tuning of experimental parameters such as temperatureand contacting time to achieve optimal conversion and selectivity.Conversions as high as 95% have been achieved in the present invention.

The present process is suitable for processing fuels having sulfurcontent ranging from several hundred to several thousand parts permillion (ppm) by weight, effectively reducing the sulfur content to lessthan 100 ppm. Sulfur content of a fuel that is to be treated may vary,depending for example on the geographical location from which theoriginal crude oil is obtained, as well as the type of fuel treated(e.g. whether the fuel is cracked or straight run). Depending on thesulfur level of the fuel to be treated, the present invention issufficiently versatile to be implemented as a primary desulfurisationprocess or as a secondary desulfurisation process for treating fuels.Non-limiting examples of fuels which can be treated by this inventioninclude gasoline, kerosene, diesel, jet fuel, furnace oils, lube oilsand residual oils. Additionally, the fuels that can be processed are notlimited to straight-run fractions, i.e. fractions obtained directly fromatmospheric or vacuum distillation in refineries, but include crackedfuels and residues which are obtained from catalytic cracking of heavycrude oil fractions. As a primary desulfurisation unit, the inventioncan substitute conventional HDS processes to process straight-run fuelswhich typically have high sulfur content of several thousand ppm, evenup to 10000 ppm (1%) or more. As a secondary desulfurisation unit, thepresent invention can be used for treating fuels that have beenundergone HDS treatment and thus have sulfur content of 500 ppm or less.In one embodiment, HDS is first carried out to lower sulfur content tothe range of about 300 to 800 ppm. Thereafter, the process of thepresent invention can be used to further lower sulfur content to lessthan 100 ppm or even less than 50 ppm, if desired. For economic reasons,the initial removal of high levels of sulfur from fuel is more suitablycarried out by a conventional HDS process. In one embodiment, the fuelcomprises diesel that has been treated in a hydrodesulfurization (HDS)process. In general, the present process is most preferably used-forprocessing low viscosity fuels such as diesel and other fuels havingviscosities that are comparable or lower than diesel. Nevertheless, ifrequired, this process can still be applied to heavier fractions such aslube oils and residual oils.

In the context of the invention, the term ‘lowered sulfur content’refers to fuel that has sulfur content of less than 500 ppm by weight.The present invention is able to reduce sulfur content in fuels to lessthan 500 ppm, preferably less than 200 ppm, and more preferably lessthan 100 ppm, and most preferably less than 50 ppm.

Sulfur-containing compounds that are typically found in petroleumfractions and which can be removed by the process of the inventioninclude aliphatic or aromatic sulfur-containing compounds such assulfides (e.g. diphenylsulfide, dibutylsulfide, methylphenylsulfide),disulfides, and mercaptans, as well as heterocyclic sulfur-containingcompounds such as thiophene, benzothiophene (BT), dibenzothiophene(DBT), 4-methyl-dibenzothiophene (mDBT), 4,6-dimethyl-dibenzothiophene(dmDBT) and tribenzothiophene, and other derivatives thereof, forexample.

The oxidation of the above sulfur-containing compounds occur at varyingdegrees of ease. Simple sulfur-containing compounds such as aliphatic oraromatic mercaptans and sulfides are generally more easily oxidized thanheterocyclic sulfur-containing compounds. Heterocyclic compoundstypically comprise thiophenic substances such as thiophenes, BT, DBT,akylated DBTs such as 4-methyl-dibenzothiophene,4,6-dimethyl-dibenzothiophene as well as other higher boiling pointderivatives. One possible reason for the resistance to oxidation in thelatter class of sulfur-containing compounds is the shielding of thesulfur by bulky hydrocarbon structures in the molecule. This class ofsulfur-containing compounds are not easily oxidised or decoupled fromthe hydrocarbons by means of conventional HDS processes, and have thusbecome known as ‘hard’ or ‘refractory’ sulfur compounds.

The conversion of thiophenic compounds into polar sulfones and/orsulfoxides using air as oxidant is the principal reaction carried out inthe invention. The general reaction scheme for the ODS process is asfollows:

As can be seen from scheme (I), the sulfones can decompose to liberateSO₂, while leaving behind a useful hydrocarbon compounds that can beutilised.

Air is utilised in the present invention to oxidise the residual sulfurcompounds mainly into their corresponding sulfones. While it istheoretically possible that some of the thiophenic sulfur compounds maybe converted into other oxidised forms than sulfones, e.g. sulfoxides,gas chromatography data obtained from experiments according to theexamples reveal that virtually no other sulfur compounds were formed.Without wishing to be bound by theory, it is believed that the sulfoxidespecies is unstable and will be oxidised into a corresponding sulfone bythe process of the present invention. Accordingly, the present inventioncan be employed to convert sulfur compounds in fuels almost completelyinto sulfones, which can subsequently be extracted in a convenientmanner. The oxidation of specific sulfur-containing compounds,particularly thiophenic compounds such as BT and DBT, which the presentinvention is effective in carrying out, is shown in the followingillustrative reaction schemes:

It can be seen from the reversible scheme (III) that the S═O bonds canbe polarised due to the loss of an electron from the sulfur atom to thepair of electronegative double-bonded oxygen atoms. It is probable thatthese polar compounds do not exist in a single form, i.e. either as annon-polarised sulfone or a fully polarised compounds, but rather ascompounds having an intermediate range of dipole moment values. As mostof the other liquid phase components in the reaction mixture arenon-polar in nature, the polar sulfone compounds can be easily separatedusing conventional separation methods such as solvent extraction oradsorption.

The contacting of fuel with air can typically be carried out in anysuitable continuous flow or batch reactor. Suitable continuous-flowreactors can, for example, be any commercially available tubular orpacked-bed column reactor. Typical single fixed bed catalyst packingconfigurations found in hydrodesulfurisation processes can be used inthe present invention. In order to provide uniform distribution of thecatalyst in the reactor (thereby ensuring uniform temperature profileand gas pressure drop through the catalyst with no hot spots), thetransition metal oxide catalyst can be held in any commerciallyavailable structured packing that can improve contact between the fuel,air and the metal oxide catalyst. The treated fuel leaving the ODSreactor contains both desulfurised fuel and oxidised sulfur compoundswhich can be readily separated by means of any suitable separationprocess such as solvent extraction or distillation. If a batch reactoris used, a fixed amount of fuel can be placed in the batch reactor whileair is bubbled into the fuel. Once the reaction is complete, theoxidised sulfur compounds may be separated from the treated fuel usingany suitable separation technique. If desired, treated fuel may beprocessed in a second run of the oxidation process to further reducesulfur content in the fuel.

Generally, the contacting of fuel with air is carried out at atemperature range of between 90° C. to 250° C., more preferably from 90°C. to 200° C. The choice of the reaction temperature is typicallyinfluenced by factors such as the boiling range of the fuel beingtreated and the desired level of conversion. The boiling point of fuelsthat can be processed typically range from less than 100° C. to severalhundred degrees Celsius. For example, if the boiling range of the fuelis above 180° C., a reaction temperature range of 130° C. to 180° C. isused. Fuels having such a boiling range include kerosene, diesel, gasoil and heavy gas oils. As noted above, one advantage of the presentinvention is that the treatment of fuel takes place in the liquid phase,meaning that the contacting generally takes place at temperatures lowerthan the boiling range of the fuel for a given reaction pressure. It isknown that an elevated reaction temperature is desirable for improvingthe kinetics of the oxidation reaction, thereby obtaining higherconversion levels. However, due to the exothermicity of the oxidationreaction, high temperatures can be inhibitory from a thermodynamicviewpoint. Furthermore, an elevated temperature is associated withunwanted side reactions that can result in the formation of undesirablepolymers and coke. Accordingly, an optimal reaction temperature rangethat takes into consideration these opposing factors would be beneficialin carrying out the invention.

In one embodiment, the contacting of fuel with air is carried out at atemperature range of about 110° C. to 190° C., and preferably between130° C. and 180° C., and more preferably between 130° C. to 160° C. Aparticularly preferred temperature range is between 130° C. and 150° C.,including about 130° C. to 140° C., or even more preferably about 140°C. Accordingly, in some particularly preferred embodiments in whichdiesel fuel is treated and supported cobalt or manganese oxide catalystsare used, a preferred reaction temperature is about 150° C. In anotherparticularly preferred embodiment, a preferred reaction temperature isabout 130° C.

The pressure at which contacting is carried out should generally be low,but at the same time sufficiently high to avoid flashing of the fuel inthe reactor that is lost with the effluent air. In general, reactionpressures that are typically used in the invention may be about 1 bar ormay range from less than 1 bar to slightly above 1 bar (about 1.2 bar)or about 2.5 bars or about 5 bars. Carrying out the oxidation reactionat elevated reaction pressures may be advantageous as the elevatedpressure may improve the oxidant concentration in the reaction system.In a preferred embodiment, the contacting takes place at ambientpressure, meaning at about 1 bar.

Notwithstanding the fact that certain temperatures ranges and pressuresare preferred in specific embodiments, it should be noted that in thebroad practice of the invention, the oxidation of sulfur containingcompounds present in fuel can be achieved even if reaction temperaturesand pressures falling outside the above preferred ranges are used,though the conversion rate may not be optimal in such cases.

The oxidation reaction which is carried out in the present inventioninvolves the use of air as the (sole) oxidant for carrying out theoxidation of sulfur-containing compounds in the fuel. It is noted thatthe term “air” as used herein is to be understood in its regularmeaning. The term thus refers to a mixture of atmospheric gasescomprising gases such as nitrogen, oxygen, carbon dioxide, trace amountsof other gases and optionally also water vapour. Gaseous oxygen isinvolved in the oxidation of the sulfur-containing compounds, whileother gases such as nitrogen passes through the reactor without beinginvolved in any reaction, given the mild reacting conditions of theprocess. In this respect, the oxygen content in air is typically knownto be about 21% by volume, although this level of oxygen may vary.Accordingly, the oxygen content of air that is used here may be at aboutits regular level in the atmosphere, i.e. 21%. It may, however, also belower, e.g. if oxygen depleted air is used, or may be higher, if oxygenenriched air is used. Depending of the oxygen level, the flowrate of airinto the reaction environment can be adjusted dynamically byimplementing a conventional feedback control, based for example based onthe measured oxygen content of the air introduced into the reactor.Alternatively, instead of adjusting the flowrate of air into thereaction environment, the reaction environment can be dynamicallysupplemented with a high purity oxygen stream using a feedback control.

The present invention makes use of a transition metal oxide catalyst forthe oxidation of the sulfur-containing compounds. In the presentinvention, any transition metal oxide exhibiting catalytic activitytowards the oxidation of sulfur compounds, preferably hard sulfurcompounds such as thiophenic compounds and the higher homologs thereof,may be used in the invention. Examples of suitable catalytic transitionmetal oxides include but is not limited to oxides of transition metalssuch as vanadium, chromium, manganese, cobalt, nickel, zirconium,niobium, molybdenum, rhenium, tantalum, and tungsten. Specific examplesof transition metal oxides include MnO₂, Cr₂O₃, V₂O₅, NiO₂, MoO₃ andCo₃O₄. Chromates, vanadates, manganates, rhenates, molybdates andniobates of the transition metal may also be used as catalyst. Dependingon factors such as cost and availability, preferred transition metaloxides are those that exhibit highly catalytic activity towards theselective oxidation of sulfur containing compounds, especiallythiophenic compounds.

In one embodiment, the transition metal oxide is an oxide of a metalselected from Groups 6, 7, 8 or 9 of the Periodic Table (IUPAC 1990),with oxides of manganese, cobalt, iron and chromium being presentlypreferred in the invention. In addition, the catalyst may comprise asingle transition metal oxide or a mixture of transition metal oxides.The transition metal oxide catalyst can be present in a single or inmultiple oxidation states.

Solid catalysts are preferably used in the invention. The catalyst canbe present in any useable form, such as powders, pellets, extrudedstructures, monoliths or crushed structures, for example. Conventionaltechniques can be used prepare the catalysts in the desired form for usein the present invention. For example, in order to prepare powdercatalysts, it is possible to calcine the corresponding metal nitrates ormetal acetates under static air for 3 hours, using a calcinationtemperature in the range of 500-600° C. in order to obtain the metaloxides. The heating rate can be pre-determined by thermal gravimetricanalysis.

In certain embodiments of the invention, solid catalysts are preferablyemployed in the form of porous pellets. Porous catalyst pellets arecommonly known and can be produced according to any conventional method.For example, it is possible to mix the catalyst components into a pasteand extrude the paste as pellets, which are then baked at a hightemperature. In order to obtain supported catalysts, it is possible todope a support pellet with the transition metal oxide catalyst byimmersing the support pellet in a salt solution of the transition metal.Additionally, pellets can adopt any suitable shape, including pelletsthat are spherical, cylindrical, star shaped or ring shaped, forexample.

In one embodiment of the invention, the catalyst used ismounted/supported on a porous support. Supported catalysts are typicallyporous pellets having catalytic material deposited as a thin film ontoits surface. The porous support can comprise a chemically inert materialhaving no effect on the oxidation reaction, or it can comprise amaterial that exerts a promoting effect on the catalyst which itsupports, thereby improving the oxidation ability of the catalyst, e.g.silica carrier promotes chromia catalyst. Whilst catalyst pellets cancomprise solely of catalytic material, it is usually not economicallyattractive since a substantial mass of catalytic material remains lockedwithin the pellet and is thus not effectively exposed for contact withreactants.

The use of a porous support helps to increase the surface area to volumeratio of the supported catalyst, thus providing a larger surface areafor the oxidation reaction to take place. For this purpose, any varietyof porous support may be used, including microporous (d<2 nm),mesoporous (2<d<50 nm) and macroporous (d<50 nm) supports. Materialswhich can be used as the porous support include metal oxides such astitania, alumina, ceria, magnesia, zirconia and tin oxide. Refractorymaterials that can withstand high reaction temperatures, such as ceramicmaterials, can also be used, and examples include silica or aluminabased ceramic materials. Other suitable materials include activatedcarbon, as well as members of the zeolite mineral group, for instanceY-zeolites, mordenite, clinoptilolite, chabazite, and phillipsite. It ispresently possible that the support comprises one single material or amixture or combination of several materials, such as amorphoussilica-alumina.

In one embodiment in which manganese and/or cobalt oxide is used as thecatalytic material, the support comprises aluminium oxide (alumina),preferably γ-alumina. Alumina supports can be in the form of pellets orextrudates, and can be obtained by any conventional method, such as dropcoagulation of an alumina suspension, or via agglomeration.

Specific combinations of catalyst and support that are suitable for usein the invention include CoO/Al₂O₃, Co₃O₄/Al₂O₃, MnO₂/Al₂O₃,Mn₂O₃/Al₂O₃, CoO;Co₃O₄/Al₂O₃, Co₃O₄;MnO₂/Al₂O₃, CoO;MnO₂/Al₂O₃,CoO/SiO₂, Co₃O₄/SiO₂, MnO₂/SiO₂, Mn₂O₃/SiO₂, CoO;Co₃O₄/SiO₂,Co₃O₄;MnO₂/SiO₂, CoO;MnO₂/SiO₂, MoO₂/Al₂O₃, MoO₃/Al₂O₃, Ru/SiO₂,Mg;Al/SiO₂, Co;Al/SiO₂, Ni/SiO₂, or Co;Ni/Al₂O₃, for example.

Apart from the selection of transition metal oxides to use as thecatalyst, the choice of a suitable catalyst loading level can help tocontribute to achieving an optimal oxidation of the sulfur-containingcompounds. In this context, catalyst loading is defined as the weightpercentage of transition metal oxide present with respect to thesupport, preferably with respect to the weight of the support beforeloading the support with the catalyst. Generally speaking, catalystloading can be determined once calcination has been carried out on thecatalyst in which the transition metal salt is converted into thecorresponding transition metal oxide. For the ease of calculation of thecatalyst loading, it is assumed in the present invention that therespective metal will be present after calcination as a homogenous oxidewith a uniform oxidation state, for example as MnO₂, NiO₂, or Co₃O₄.Inductively coupled plasma spectroscopy (ICP) measurements can be madeto determine the metal concentration in the catalysts. From that ICPmeasurements, the actual percentage of the metal oxide present can becalculated. Apart from ICP, the prepared catalysts can also be analysedby Scanning Electron Microscopy (SEM) energy dispersive analysis byX-Ray (EDAX), which will give the surface composition of the catalyst.Loading levels that fall below the optimal range (which can bedetermined empirically by the skilled person), may result in loweryields, while loading levels that are increased above the empiricallydetermined optimal range may provide diminishing returns in terms ofconversion. In one embodiment of the invention, the catalyst loading isin the range of 1 to 17%, more preferably between 2 to 13%, of theweight of the support used. It should be noted that other catalystloading values falling outside this range can nevertheless be used, eventhough they may be less than optimal and may thus place compensatorydemands on other areas of the process. For example, if a low loadinglevel is used, the corresponding low conversion of the sulfur-containingcompounds may necessitate higher space time, temperature or pressure,consequently leading to increased reactor size or possible unwanted sidereactions, respectively.

Where the catalysts used in the invention are to be mounted ontosupports, any conventional impregnation method known in the art may beused to prepare the catalysts. Such methods include incipient wetness,adsorption, deposition and grafting. If the incipient wetness method isused, for example, a solution containing a salt of the catalytictransition metal is first prepared. The support on which the catalyst isto be mounted may be subjected to pre-drying at elevated temperaturesovernight before impregnation. This drying step helps to remove theadsorbed moisture from the pores and to fully utilize the pores forefficient and uniform impregnation of the metal salt solution. Theconcentration of the salt solution is prepared according to the desiredcatalyst loading level. For example, in order to prepare a catalyst witha loading level of 5% MnO₂ supported on γ-alumina, that is 0.5 g of MnO₂on 10 g γ-alumina, log of pre-dried γ-alumina can be impregnated in asolution containing 1.409 g of Mn(II) acetate×4H₂O (molecular weight245.09) dissolved in 8.0 ml deionised water. As can be seen from thisexample, it is assumed for the calculation of the catalyst loading thatthe Mn salt is completely converted into MnO₂ during the subsequentcalcination and that formation of mixed metal oxides such as MnAl₂O₄ canbe neglected. The wetted support is subsequently left to dry. The dryingmay be carried out by baking the wetted supports in an oven to calcinethe catalyst. Calcination of the metal salt leads to the formation of alayer of metal oxide on the support.

In order to form a catalyst comprising a homogeneous mixture of two ormore transition metal oxides, it is possible to wet the supportstructures in a mixture containing the salts of two or more of thedesired transition metals. On the other hand, if it is desired todisperse several layers of different transition metal oxides on thesupport, the impregnation and baking steps can be sequentially performedwith the salt solution of each respective transition metal. In thiscontext, the salt that is used to prepare a salt solution is known asthe catalyst precursor. Suitable precursors include crystalline salts ofthe transition metal such as nitrates, chlorides, sulphates, bromides,iodides, phosphates, carbonates, as well as organic compounds of themetals, such as acetates, benzoates, acrylates and alkoxides. It shouldbe noted that in order to form a solution using these salts, they shouldbe water soluble or soluble in an organic solvent. Methods of preparingsuitable supported or bulk catalysts for use in the present inventionare described in Example 1 as well as taught in WO 03/051798 and thereferences cited therein, for example.

It is also contemplated that the catalyst formulation can additionallyinclude other components, such as promoters which can enhance catalystactivity or prolong the process lifespan of the catalyst. It may also bedesirable that the catalysts are presulfided before use.

The process of the present invention may be supplemented by othersuitable pre- or post-treatment steps. For example, the fuel to betreated can be subjected to prior chemical or thermal treatment beforeit is contacted with air. It is also possible to pre-heat the processair prior to introducing the air into the reactor. Once the contactinghas been performed, it is also possible to carry out a varietypost-processing steps, such as separation steps to separate the oxidisedsulfur compounds from the fuel or to remove any sulfur dioxide gas fromthe exhaust air prior to releasing it into the atmosphere.

In order to remove the oxidised sulfur compounds, of which a largepercentage comprises sulfones, from the treated fuel, the polarity ofthe sulfone molecule is relied upon to extract the sulfones from thehydrocarbon organic phase into aqueous phase. Thus, one embodiment ofthe present invention further comprises adding a polar organic solventto the treated fuel after contacting with air, thereby extracting theoxidised sulfur-containing compounds from the treated fuel, andseparating the polar organic solvent and the oxidised sulfur-containingcompounds from the treated fuel. This embodiment is based onliquid-liquid extraction using polar solvents that are insoluble in thehydrocarbon fuel. The choice of solvent is influenced by severalfactors, such as selectivity of the oxidised sulfur compounds in thesolvent, density of the solvent, insolubility of the solvent in thetreated fuel, and recoverability of the solvent. One factor to considerin choosing a solvent is the selectivity of the solvent towards thepolar oxidised sulfur-containing compounds. Typically, organic compoundshaving high polarity, as observed from their Hildebrand's solubilityparameter, are selective towards the solvation of the oxidised sulfurcompounds. Selectivity of extraction is important because the extractionof valuable carbonyl and aromatic hydrocarbons from the fuel should beminimised. Apart from this consideration, the selected fuel shouldpreferably also be one that is immiscible (partition coefficient) in thefuel and has a different density from the treated fuel, so that thefuel/solvent mixture can be easily separated by conventional means suchas gravity separation or centrifugation. It may also be helpful tochoose a solvent that has a boiling point that is different from theboiling point of the sulfones to be extracted, so that distillation canbe readily carried out to separate the sulfones from the solventsubsequently.

Various types of equipment can be used for solvent extraction, and itsselection can depend on factors such as cost, size of equipment orprocess throughput, for example. When carrying out large scale solventextraction of the oxidised sulfur compounds, a single stagemixer-settlers can be used, or if better extraction is desired,multi-stage cascades may be used instead. Alternatively, sieve trayextraction towers may also be used.

In one embodiment of the extracting step, between about 1 to 4 parts byvolume of fuel is contacted with about 1 part by volume of polar organicsolvent. The quantity of solvent used in solvent extraction affects theextent of extraction. While increasing the quantity of solvent improvesthe extraction of the oxidised sulfur compounds from the fuel, thisadvantage is counteracted by other considerations such as increasedcosts due to the larger amounts of solvent being used as well asincrease in the scale of solvent recovery operations.

Numerous polar organic substances can be used for the solvent extractionof the oxidised sulfur compounds. These include acetonitrile (AcN),dimethyl sulfoxide, N,N′-dimethyl-acetamide, N-methyl-pyrolidinone,trimethylphosphate, hexamethylphosphoric amide, methanol (MeOH),ethanol, propanol, butanol, carbon disulfide, pyridine, propyleneglycol, ethylene glycol or any mixture thereof etc. In one embodiment,the polar organic solvent comprises N,N′-dimethyl-formamide (DMF),1-methyl-2-pyrrolidone (NMP), acetone or any mixture thereof. Thesolvent can also be diluted with water, if desired.

In general, the polar organic solvent and the dissolved oxidised sulfurcompounds can be separated from the fuel by gravity separation orcentrifuging. The organic solvent can subsequently be recovered usingany conventional separation method, such as evaporation, distillation orchromatography, to recover the solvent for recycle. The desulphurisedfuel can be further processed, such as by washing with water oradsorption using silica gel or alumina, to remove traces of the solvent.The fuel thus obtained has sulfur-content of typically less than 100ppm, or preferably less than 50 ppm.

In one embodiment of the invention, the treated fuel is contacted with abasic adsorbent. The basic adsorbents used herein should exhibit atendency towards the preferential adsorption of any acidic speciespresent in the fuel. The contacting step in this embodiment can beadvantageously carried out after the separation/extraction step toeliminate remaining traces of the sulfones in the fuel. As sulfones areweakly acidic in nature, the use of a basic adsorbent can remove them aswell as other acidic impurities such as other sulfur-based ornitrogen-based impurities from the fuel. Examples of such basicadsorbents include zeolites, activated carbon, and layered-doublehydroxides (LDH). LDHs are preferably used in some embodiments andexamples of suitable LDHs include those based on the metals Mn, Co, Ni,Cr, Al, Mg, Cu, Zn and Zr coupled with exchangeable anions such as NO₃⁻, CO₃ ²⁻ and/or Cl⁻, for example. The adsorption process can be carriedout in any suitable furnace reactor, such as in a continuous flow tubefurnace with the absorbent packed as a fixed bed. In order to regeneratethe adsorbent, a base can be added to the adsorption column toregenerate the adsorbent. The overall recovery that can be achieved witha combination of solvent extraction and adsorption can be as high as92%.

The invention will be further explained by the following non-limitingexamples and the accompanying figures, in which:

FIG. 1 shows the simplified process flowsheet of the oxidativedesulfurisation (ODS) process according to the invention.

FIG. 2 shows the process flowsheet of a specific embodiment of the ODSprocess according to the present invention. In this embodiment, ODS iscarried out as a secondary desulfurisation process for fuels that havebeen treated by conventional HDS. The treated fuel is channelled to astirred/mixing tank containing a solvent for removing the oxidisedsulfur compounds. The fuel/solvent mixture is then channelled to asettler where the treated fuel is separated from the solvent.

FIG. 3 shows another embodiment of the process shown in FIG. 2, in whichthe treated fuel is further passed through basic adsorbent column forfurther removal of the remaining sulfur-containing (which is slightlyacidic in nature) compounds in the fuel. The fuel passing out of theadsorption column is sulfur-free.

FIG. 4 shows the results of the analysis of the prepared catalysts basedon the Brunauer, Emmett and Teller (BET) method.

FIGS. 5A to 5D show the results of analysis carried out with a gaschromatography Flame Ionisation Detector (GC-FID) on model diesel beforeoxidation was carried out (a) and after oxidation was carried out usingthe present invention (b). After solvent extraction using NMP wasperformed, the fuel and the solvent layers were each analysed. Figures(c) and (d) shows the analysis results of the n-tetradecane layer theNMP layer, respectively.

FIGS. 6A to 6H show the individual gas chromatograms of specific samplesof treated model diesel. In the experiments carried out for the resultsshown in FIGS. 6A & B, the catalyst used was 5% MnO₂/γ-alumina.Treatment temperature was 130° C. FIG. 6A shows the analysis resultbefore treatment, while FIG. 6B shows the analysis result aftertreatment. FIGS. 6C & 6D show the GC results of model diesel treated inthe absence of catalyst at a temperature of 130° C., before treatmentand after 18 hours of treatment, respectively. No oxidation wasobserved. FIGS. 6E & 6F show the GC analysis results of model dieseltreated with 5% MnO₂/γ-alumina catalyst at a temperature of 150° C.,before treatment and after 18 hours of treatment, respectively. FIG. 6G& 6H show the GC analysis results of model diesel treated with 8%MnO₂/γ-alumina catalyst at a temp. 150° C., before treatment and after18 hours of treatment, respectively.

FIG. 7 shows the conversion of DBT vs. time in model diesel at 130° C.for manganese (▪)- and cobalt (♦)-containing catalysts.

FIG. 8A shows the gas chromatography-atomic emission detection (GC-AED)chromatogram of untreated real diesel used in the examples. FIG. 8Bshows a table of data from X-ray florescence (XFR) analysis of sulfurcontent in untreated diesel that has undergone only solvent extraction.

FIG. 9 shows a table of data from XRF analysis of sulfur content in realdiesel that has been treated with either Co₃O₄ or MnO₂ catalystsupported on γ-alumina, and solvent extraction carried out with AcN,DMF, NMP and methanol. Treatment temperature was about 130° C.

FIG. 10 shows a table of data from XRF analysis of sulfur content inreal diesel that has been treated with MnO₂ catalyst supported onγ-alumina, and single or multiple solvent extraction carried out withAcN, DMF, NMP and methanol. Treatment temperature was either 130° C. or150° C.

FIGS. 11A to 11C show sulfur AED chromatograms of treated samples markedwith superscript 3Ci, 3Cii and 3Ciii in the table in FIG. 10.

FIG. 12 shows a table of data from XRF analysis of sulfur content inreal diesel that has been treated with MnO₂ catalyst supported onγ-alumina. Comparisons can be made between the effectiveness of sulfurremoval employing a single solvent extraction using NMP and withoutemploying any solvent extraction step. Treatment temperature was at 150°C. The initial sulfur content of the real diesel was 440-454 ppm. Sulfurcontent measurements were taken by ASTM 2622 (Brucker XRF).

FIG. 13 shows the graph of sulfur content in a treated fuel sample vsratio of solvent to diesel fuel applied in the solvent extractionprocess. It will be noted that sulfur content is generally reduced assolvent to fuel ratio is increased.

EXAMPLE 1 Catalyst Preparation and Characterization

The catalysts to be prepared comprise transition metal oxides and poroussupport with high specific surface area have been prepared byimpregnation using incipient wetness method. 10 g of γ-alumina pellet(diameter=3-4 mm, length=6-10 mm, specific surface area=370 m²/g,specific pore volume ranged from 0.82 ml/g to 0.87 ml/g) was impregnatedwith cobalt nitrate and/or manganese acetate aqueous solutions. Thetotal metal oxides loading with respect to γ-alumina ranged from 2 to 13wt %. The impregnated sample was left on the roller which was set at 25rpm for approximately 18 h to obtain better dispersion. The sample wasthen dried at 120° C. in the oven for 18 h for removal of the watercontent. The dried sample was calcined in a static furnace at 550° C.for 5 hours with a ramp of 5° C./min. Powder X-ray diffraction (XRD)showed that the catalysts were amorphous and that no distinguishablecrystallographic properties could be observed among the catalysts. Theprepared catalysts were also characterised by N₂ adsorption/desorption,and thermogravimetric analysis (TGA) in order to obtain the informationon surface area, pore size distribution and pore volume, crystallographyand thermal decomposition of the samples. The BET method of measurementwere used to determine the catalyst surface area. The characterisationdata for the prepared catalysts used in the subsequent examples aresummed up in the table in FIG. 4.

EXAMPLE 2 Oxidative Desulfurisation with Solvent Extraction Using aModel Diesel

DBT and/or 4-MDBT were chosen to prepare model diesel by dissolving themin n-tetradecane with a total sulphur content of 500-800 ppm. In most ofthe experiments, sulfur content in the model diesel was introduced byadding only DBT. In the remaining experiments, both 4-MDBT and DBT wereadded. The oxidation experiments were carried out in a stirred batchreactor.

In a two-necked round bottom flask, 10.0 ml of model diesel containingapproximately 500 ppm of sulphur underwent oxidative reaction in thepresence of 20-30 mg of the catalyst (diameter=3-4 mm, length=6-10 mm).The mixture was magnetically stirred to ensure a good mixing and bubbledwith purified air at flow of 60 ml/min. The reactions were carried outat a temperature range of 90-200° C. The optimum temperature for thisspecific set up was found to be 130° C. at which the oxidation of themodel compounds occurred successfully with insignificant side-reactionof solvent oxidation. A water-cooled reflux condenser was mounted on topof the reaction flask to prevent solvent loss and also function as anoutlet for air.

At different time intervals (3h), 50 μl of the reacted diesel waswithdrawn and diluted with 500 μl of diethylether for gas chromatographyanalysis. After the oxidation reaction, the oxidised products in themodel diesel were extracted with polar organic solvents such asmethanol, N,N-dimethylformamide (DMF), acetonitrile (AcN) and1-methyl-2-pyrrolidone (NMP). During this process, the reacted modeldiesel was mixed with these polar organic solvents at different volumeratios (e.g. organic phase: polar solvent=4:1 as shown in FIG. 5D) andwas magnetically stirred vigorously for 1 h. The mixture was thentransferred into a separating funnel for the model diesel and polarorganic solvent to be separated into different layers. The thus-treatedmodel diesel was analysed with GC. The sulphur-containing polar solventlayer was then collected and analysed by GC. In the case of usingmethanol, the methanol solvent was removed by the rotary evaporator. Theremaining solid product was collected and analysed by the GC afterre-dissolving into methanol or NMP (1-methyl-2-pyrrolidone) solvent.

FIGS. 5A to 5D shows the results of sulfur analysis from a gaschromatography-atomic emission detector (GC-FID) of the model dieselbefore and after the oxidative process of the present invention carriedout on model diesel. As shown in the results, almost complete conversionof DBT to the corresponding sulfone was achieved (cf. FIG. 5A and 5B). Asmall percentage (about 5%) of n-tetradecane was oxidised to6-tetradecanone, 2-tetradecanone and 4-tetradecanol. These are termedoxygenates and are known to enhance diesel quality. It was found thatNMP and DMF were better solvents than methanol and AcN. NMP solventextraction achieved almost complete removal of the sulfones (cf. FIGS.5C and 5D, in which a diesel:solvent volume ratio of about 4:1 wasused). Additionally, multiple extractions were found to be better than asingle extraction.

In a further experiment, specific samples of the model diesel weretreated with different MnO₂ catalysts having different catalyst loadinglevels, and at temperatures of either 130° C. or 150° C. The treateddiesel samples were analysed with gas chromatography (GC-FID) before thestart of the oxidative treatment and after 18 hours of reaction time inorder to determine the catalytic activity of the catalyst for oxidationreaction using air as oxidant at 130° C. (FIG. 6A & 6B). In a similarexperiment carried out without catalyst, it was observed that thereaction could not proceed (FIG. 6C & 6D). The result of the analysisare shown in FIGS. 6A to 6H. In summary, FIGS. 6A-6D show that thecatalyst is important for the selective oxidation of dibenzothiophene tocorresponding sulfone at 130° C. FIGS. 6E-6H further show that thecatalytic activity of 5-8% MnO2 loaded on gamma alumina for model dieseland a reaction temperature of 150° C. provide advantageous conditionsfor selective oxidation of dibenzothiophene without oxidising thehydrocarbons such as tetradecane or pentadecane.

As can be seen from FIG. 7 showing the conversion of DBT throughout theoxidative treatment, conversion reached above 90% between the reactiontime of 15hr to 18hr.

EXAMPLE 3 Oxidative Desulfurisation and Solvent Extraction on RealDiesel

A) Solvent Extraction on Diesel Without Oxidative Treatment

Four 25.0 ml samples of untreated diesel was mixed with the polarorganic solvents AcN, DMF, NMP and MeOH, respectively, in order todetermine the effect of solvent extraction on sulfur compounds presentin untreated fuel. After extraction by the respective polar solvents,the sulfur content of the diesel was measured by X-ray florescence(XRF). Untreated diesel had sulfur content of 370-380 ppm beforeextraction was carried out (measured by XRF using s-standard calibrationcurve). The GC-AED analysis of the sulfur content in the diesel is shownin FIG. 8A. The results in FIG. 8B show that among the solvents tested,NMP was most efficient in extracting sulfur compounds present inuntreated fuel.

B) Oxidative Treatment using Co₃O₄ and MnO₂ Catalysts Supported onγ-alumina followed by solvent extraction

In a two-necked round flask, 100 ml real diesel underwent oxidativereaction in the presence of about 100 mg of catalyst. The mixture wasmagnetically stirred to ensure a good mixing and bubbled with purifiedair at flow of 60 ml/min. The reactions were carried out at 130° C. Thereaction was stopped after about 18 hours. The oxidized diesel wascooled to room temperature and divided into four portions of 25 ml eachfor extraction using different solvents (different volume). The analysisresults are shown in FIG. 9. Sulfur content of the extracted oxidizedreal diesel was measured by XRF using s-standard calibration curve.Judging from this experiment, an 8% MnO₂ supported catalyst appeared tobe more effective for removing sulfur from diesel than a 2% or 5%supported MnO₂ catalyst.

C) Oxidative Treatment using MnO₂ Catalysts Supported on γ-aluminaFollowed by Single or Multiple Solvent Extraction

In a two-necked round flask, 150 ml real diesel underwent oxidativereaction in the presence of about 30 mg of catalyst. The mixture wasmagnetically stirred to ensure a good mixing and bubbled with purifiedair at flow of 60 ml/min. The reactions were carried out at atemperature of either 130° C. or 150° C. The reaction was stopped afterabout 18 hours. The oxidized diesel was cooled to room temperature anddivided into five portions of 30 ml each for extraction using differentsolvents (different volume) via either single or multiple solventextraction,

The analysis results are shown in FIG. 10. Sulfur-content of theextracted oxidized real diesel was measured by XRF using s-standardcalibration curve. Sulfur ppm levels indicated within the brackets ( )were measured using Antek 9000S (Singapore Catalyst Centre) ASTM D-5453method. It can be seen that at a treatment temperature of 130° C., MnO₂supported catalysts provided better sulfur removal at a loading level of5% than at a loading level of 2%. Oxidative treatment carried out at atemperature of 150° C. and using catalysts at a loading level of 8%provided better sulfur removal than treatments carried out at 130° C.using catalysts having lower loading levels. Additionally, multiplesolvent extractions were able to provide better sulfur removal thansingle solvent extractions.

Sulfur AED chromatograms were also obtained for specific treated samples(marked with superscript 3Ci, 3Cii and 3Ciii in the above figure) andare shown in FIGS. 11A to 11C.

D) Effect of Solvent Extraction on Sulfur Removal after Carrying outOxidative Treatment using MnO₂ Catalysts Supported on γ-alumina

In a two-necked round flask, 150 ml real diesel underwent oxidativereaction in the presence of various amounts of catalyst. The mixture wasmagnetically stirred to ensure good mixing and bubbled with purified airat flow of 60 m/min. The reactions were carried out at 150° C. for aperiod of about 24 hours. The oxidized diesel was cooled to roomtemperature and divided into five portions of 30 ml each. Each 30 mlportion was divided into two portions. One portion of each oxidizeddiesel sample was analysed after oxidative treatment but prior tosolvent extraction to determine the amount of SO₂ (gas) released duringthe oxidation process. The other portion of each of the samplesunderwent solvent extraction using 50 ml of a respective solvent. andthen analysed for sulfur content (Bruker XRF using S-standardlessmethod, ASTM 2622).

Based on the results shown in FIG. 12, it can be seen that at aoxidation temperature of 150° C., sulfur removal provided by MnO₂supported catalysts was most effective at a loading level of 8%, ascompared to other loading levels of 5%, 11% or 13%.

1. A process for removing sulfur-containing compounds from fuel, saidprocess comprising: contacting the fuel in liquid phase with air tooxidise the sulfur-containing compounds, said contacting being carriedout in the presence of at least one transition metal oxide catalyst. 2.The process of claims 1, wherein said contacting is carried out at atemperature range of between about 90° C. to 250° C.
 3. The process ofclaim 1 or 2, wherein said contacting is carried out at a temperaturerange of between about 110° C. to 190° C.
 4. The process of any one ofclaims 1 to 3, wherein said contacting is carried out at a temperaturerange of between about 130° C. to 180° C.
 5. The process of any one ofclaims 1 to 4, wherein said contacting is carried out at a temperaturerange of between about 130° C. to 160° C.
 6. The process of any one ofclaims 1 to 5, wherein said contacting is carried out at a pressure ofabout 1 bar.
 7. The process of any one of claims 1 to 6, wherein thecatalyst is supported on a porous support.
 8. The process of claim 7,wherein the amount of catalyst supported on the porous support (catalystloading) is in the range of about 1% to 17% by weight of the poroussupport.
 9. The process of claim 7, wherein the amount of catalystsupported on the porous support (catalyst loading) is in the range of 2%to 13% by weight of the porous support.
 10. The process of any one ofclaims 7 to 9, wherein the porous support comprises γ-alumina.
 11. Theprocess of any one of claims 1 to 10, wherein the transition metal isselected from Groups 6, 7, 8 or 9 of the Periodic Table (IUPAC 1990).12. The process of claim 11, wherein the transition metal is selectedfrom the group consisting of manganese, cobalt, iron, chromium andmolybdenum.
 13. The process of any one of claims 1 to 12, furthercomprising: adding a polar organic solvent to the treated fuel aftercontacting the fuel with air, thereby extracting the oxidisedsulfur-containing compounds from the treated fuel, and separating thepolar organic solvent and the oxidised sulfur-containing compounds fromthe treated fuel.
 14. The process of claim 13, wherein the polar organicsolvent comprises acetonitrile, N,N′-dimethyl-acetamide,N-methyl-pyrolidinone, trimethylphosphate, hexamethylphosphoric amide,methanol, ethanol, propanol, butanol, pyridine, propylene glycol,ethylene glycol, N,N′-dimethyl-formamide, 1-methyl-2-pyrrolidone,acetone and mixtures thereof.
 15. The process of claim 13 or 14, wherein1 part by volume of polar organic solvent is added to between about 1 to4 parts by volume of treated fuel.
 16. The process of any one of claims1 to 15, further comprising treating the treated fuel with a basicadsorbent.
 17. The process of claim 16, wherein the basic adsorbent isselected from the group consisting of zeolites, activated carbon, andlayered-double hydroxides (LDH).
 18. The process of claim 16, furthercomprising washing the basic adsorbent with a basic solution toregenerate the basic adsorbent.
 19. The process of any one of claims 1to 17, wherein the untreated fuel comprises sulfur content in the rangeof between about 300 to 800 ppm.
 20. The process of any one of claims 1to 18, wherein the fuel is diesel that has been treated in ahydro-desulfurisation process.
 21. The process of any one of claims 1 to19, wherein the sulfur-containing compounds in the fuel comprisethiophenic compounds.
 22. The process of claim 20, wherein thethiophenic compounds are selected from the group consisting ofthiophene, benzothiophene, dibenzothiophene, 4-methyl-dibenzothiophene,4,6-dimethyl-dibenzothiophene and tribenzothiophene, and mono-, di-,tri-, and tetra-substituted compounds thereof.